This could be cause by a plugin conflict.
Please use the following troubleshooting steps.

1) If you have a cache plugin, Please clear it’s cache content and disable it.
2) Proceed to disable one plugin at a time, clear browser history, before re-visiting your site to check your menu.
3) Do plugin disabling until you find the plugin that’s causing this issue, Please remove the plugin.

You can also check if it’s cause by your custom css.
If you have any Custom CSS code in Theme Options -> Custom CSS
Please copy it and save in a text file as backup, remove it from your site and check if it fixes your issue.

If you are using a child theme with custom css, Please activate your parent theme and see if it fixes your issue.